1. Field of the Invention
The present invention relates to dobbies of the rotary type for a weaving loom and more particularly to the modulator mechanisms interposed between the shaft of the loom and the principal shaft of the dobby in order to intermittently drive the dobby with a stop every 180.degree..
2. History of the Related Art
Different types of modulator mechanisms for rotary dobbies have been proposed, certain known constructions faithfully reproducing intermittent drive systems used in other technical sectors.
Reference will firstly be made to French Patent No. 2 478 143 to STAUBLI, of which the mechanism is schematically recalled in FIG. 1 of the accompanying drawings. The mechanism described essentially comprises a rotating cage which is formed by an entry plate 1 and a disc 2 connected to each other by two longitudinal pins 3 and which is mounted idly on the principal shaft 4 of the dobby, being driven in rotation by the shaft of the weaving loom. The shaft 4 freely traverses a fixed cam 5 of the complementary type of which the two constituent discs are rigidly fixed to the housing of the dobby; against this cam 5 are applied two paris of rollers 6, each pair being mounted on a support 7 which oscillates freely on one of the two pins 3. The articulated link between each oscillating support 7 and the shaft 4 is ensured by a block 8 which moves inside a radial slideway 7a of the support and which is pivotally carried by two webs 9 fixed with shaft 4 so has to act as a drive rod.
Another known type of modulator mechanism is illustrated in U.S. Pat. No. 3,407,678 to STEINKE, and is schematically recalled in FIG. 2 of the accompanying drawings. Here, the control rollers are replaced by pinions 16 which are idly mounted on shafts 13 rotatably supported by a plate 12, which is fitted on the shaft 11 of the loom. Each pinion 16 meshes with a ring gear 15 rigidly fixed to the fixed housing of the dobby, so that each shaft 13 is driven in rotation. Each shaft 13 comprises an extension 17 offset with respect to the axis thereof, and the link between each extension 17 and the principal shaft 14 of the dobby is provided by a connecting rod 18 coupled on a web 19 fitted on the shaft 14.
More recently, an arrangement has been proposed which in fact derives directly from the two systems recalled above. As illustrated very schematically in FIG. 3, this arrangement again employs two pairs of rollers 26 mounted on two opposite supports 27 which oscillate about pins 23 in order that the rollers ride on a fixed complimentary cam 25. The link between each support 27 and the shaft 24 of the dobby is provided, as in the U.S. Patent mentioned hereinabove, by a connecting rod 28 coupled, on the one hand, on a lateral pin 27a of the support 27, and on the other hand, on a web 29 fitted on the shaft 24.
It will be understood that, for such an arrangement to function correctly, ensuring perfect balance of the forces which are exerted, on the two oscillating supports 27 of the system, it is imperative that the two oscillating supports and the parts which connect them to the shaft 24 of the dobby, are absolutely identical in geometry, any error, even slight, in dimensioning or geometry rendering it impossible for the four rollers 26 to ride simultaneously on the two tracks of the complementary cam 25; in that case there is only one oscillating support which is working, which creates unacceptable lack of balance and overload. Now, it is known that, in practice, it is difficult systematically to avoid any defect in mass-production.